Shaft current measurement

[appunni]

Sir,

How does the shaft current measurement is done in hydro machines? I know that the earthing brush is connected at turbine(driving) side and the guide bearings are not insulated to earth while the bearings at the other (nondriving) end is insulated to earth. My doubt is if the bearing near shaft earthing brush is not insulated how the current measurement will be accurate? The shaft current is found measuring through a ct which connected in series with earthing brush. I am referring about a hydromachine of 7.5MW capacity , horizontal shaft. Will the oil film during running make the insulation to earth in the driving end bearings? Or, is the driving end bearing is actually be insulated?
Two days ago, I saw that the shaft current values are found very low than which was flowing normally even though in the order of microamperes. When I meggered the shaft after removing the earthing brush I againg found meggervalue zero. That is why I thought that the driving end bearing would be earthed.
I request your valuable comments and advices.

with regards,
appunni

 

[jbartos]

Suggestion: Visit

http://www.memagazine.org/backissues/may00/features/jam/jam.html

for ceramic bearings
Also, there are different shaft currents in larger motors, one due to fast switching devices, IGBTs, IGCTs, etc. with their path directed to the ground. Another shaft currents circulate within the engine body, e.g. between the stator and rotor due to various motor physical asymmetries

 

[rlounsbu]

The connection from the shaft brush to ground is a second path shunting current away from the bearing path to ground, therefore it does not measure shaft current thru the bearing, but helps reduce it. One thing the measurement will tell you is if there is a problem with the insulation on the other bearing. Perhaps this is the reason for the lower megger reading, howerver you should compare the megger reading to historical data as you did with current. A 500 or 50 volt megger would break down the dielectic of a thin oil film and the resistance from the oil would be different if the lube pump was on of off and the generator was running of not. Check the Tech Manual or maintenance procedures for a procedure to check the bearing insulation and the purpose of the current measurement.

Rich L.

 

[brupp]

I am confused by the situation you describe. While the machine is running there should be an oil film which causes the driven end bearings to be somewhat insulated. The shaft grounding brushes should shunt off any shaft voltage, reducing the current through that bearing to 0 or very close to 0. If you want to megger the shaft it seems that you would have to do it while the machine is running. Else there would be no oil film around the bearing and the shaft journal would rest against the bearing face, causing a "short" to ground. Yet you have described removing the earthing brush in order to take a megger reading. For a meaningful reading of the insulation properties of the opposite end bearing I believe you would need to take the test while the machine is spinning, possibly by using the earthing brush as one of your connection points.

 

[edison123]

while meggering the bearing innsulation in sleeve bearing machines at standstill, you have to lift the shaft clear off the insulated bearing and check the bearing insulation. Else, you create a short to ground through your megger and shaft.

 

[jbartos]

Suggestion: References:

Raymond Ong, J.H.Dymond, R.D.Findlay and B.Szabados, “Significance of Joints Due to Lamination
Segmentation on Shaft Current in Induction Machines,” Proceedings 1999 IEEE Biennial International
Electrical Machines and Drives Conference, Seattle Washington, 9-12 May, 1999.

R.Ong, J.H.Dymond, J.V.Kay, R.D.Findlay and B.Szabados, “Impact of Oil Lubrication on the
Magnitude of Shaft Current in a Large Oil Ring Lubricated Induction Machine,” Conference
Proceedings ICEM’98, Istanbul, Turkey, 2-4 September, 1998.

R.Ong, J.H.Dymond, R.D.Findlay, B.Szabados, "Shaft Current Measurement of a Large Induction
Machine Using the Rogowski Coil Method", IEEE International Symposium SDEMPED'97, Carry le
Rouet, conference proceedings pg 23-26, Sep 1997.

Raymond Ong, J.H.Dymond, R.D.Findlay, B.Szabados, “Shaft Current Phenomenon: Experimental
Testing and Analysis of a Large Induction Motor,” Proc. International Association of Science and
Technology for Development, International Conference on High Technology in the Power industry,
Orlando, Florida, Paper 262-025, pp 241-6, Oct., 27-30, 1997,.